I have recently received a penalty notice for stopping in a box
junction on a busy London road intersection. I recognise that I did
stop in the box junction, for some time, and probably blocked the
crossing traffic, and I intend to pay the fine.

Dude - do what that footballer did recently and send the form back unsigned
and then contest the penalty and the points in the magistrates court.

Apparently the road traffic act does not require you to sign the form.